* Generally, density increases as water cools. This is true for most substances. As the temperature decreases, the molecules slow down and move closer together, resulting in a higher density.

* The exception: Water between 4°C and 0°C. This is where water behaves unusually. As water cools from 4°C to 0°C, its density actually *decreases*. This is because the hydrogen bonds between water molecules start to form a more open, crystalline structure, making the ice less dense than liquid water.

Key Points:

* Maximum density: Liquid water reaches its maximum density at 4°C.

* Ice floats: The lower density of ice is why it floats on water.

* Important for life: This unusual property of water is crucial for life on Earth. It allows bodies of water to freeze from the top down, providing insulation for aquatic life below.

Let me know if you have any other questions!